Practical investigations
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How do you find the percentage purity of magnesium carbonate in an indigestion tablet by (a) thermal decomposition, and (b) reacting it with an acid?
Igloo writes
In both reactions carbon dioxide is given off and so you need to use apparatus which will measure the quantity of this gas formed.
In each case you should weigh the tablet(s) being used. In the case of thermal decomposition, the best technique is to heat the crushed tablet(s) to “constant mass” before determining the loss in mass, which of course equals the mass of carbon dioxide evolved. In the reaction with acid it is probably best to measure the volume of carbon dioxide produced by carrying out the experiment in a conical flask, using an excess of dilute hydrochloric (or nitric) acid, and collecting the gas in a syringe clamped horizontally above the flask.
From the mass (or volume) of carbon dioxide given off it is then necessary to convert this to moles, and then, via the two equations, to deduce the number of moles of magnesium carbonate present in the tablet(s). By converting this to a mass in grams, and comparing this with the mass of tablet(s) originally used, it is then easy to calculate the percentage by mass of magnesium carbonate in a tablet.
It should be noted that you are assuming in these calculations that magnesium carbonate is the only type of carbonate present in the tablet.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 19 May 2005
